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The situation is you have two separate Exchange servers in two sites and they share the same public domain name.

So the question is how does the exchange server in the first site know to forward emails to the exchange server in the second site for second site user’s?

i.e., if you have user on server 1 called user1@domain.com and user on server 2 called user2@domain.com and the MX record point to server 1.

if you send mail to user1 it will arrive to his mail box but if you send to user2 it will not arrive as his mail box is not on server 1 ( the one that the MX record point to ).

So the question is how does the exchange server in the first site know to forward emails to the exchange server in the second site for second site user’s?

Here you are the solution.

Exchange Configuration

You have to choose a primary site for your public name. This primary site is the one that will receive all the email. In this example, Cairo is Primary, Alex is secondary.

Setup MX records for your primary domain pointing to these servers.

Create a recipient policy on each server for your primary domain. Make sure that the option about exchange being responsible for all email delivery to this address is enabled. It should be the primary recipient policy.

Create sub domains for each site in the DNS of each server.
Therefore if you had two sites of Cairo and Alex then you would have

Cairo.domain.com

Alex.domain.com

While working in the internal DNS of each server, create MX records with the external IP address of the other server.
Therefore the Cairo site will have DNS zones for Alex.domain.com and in this zone will be a DNS entry for mail.Alex.domain.com. Each of those would also be set as MX records.
These MX records do NOT appear on the Internet, but traffic will flow on them because your local machine is looking up the MX records from the location DNS.

On each server, add a new additional recipient policy – but don’t make it default. This new recipient policy should match the location.
Continuing with our example:

In Cairo, it would be Cairo.domain.com

In Alex it would be Alex.domain.com

The key is that it should NOT be the default policy on any site.

The result of this should be that all users have two email addresses – the default one ending in domain.com and a secondary one that ends location.domain.com.

NOTE: It is important that your DNS is configured correctly. The server should be configured to use your active directory domain controllers for DNS – no external DNS servers should be used.
If you need to use external DNS servers for performance reasons then configure these as forwarders on the active directory DNS servers.

Adding the Remote Users

On the primary server create a mail enabled contact for all users located on the other servers. When creating the contact, initially put in the email address for its home address (Cairo.domain.com). Once created, wait a moment for recipient policy to stamp the account. You should find that the contact now has two email addresses, @domain.com and @Cairo.domain.com. Do not add local users as they will already have an email address.

Repeat on the other server.

Cairo will have mail enabled contacts for Alex.

Alex will have mail enabled contacts for Cairo.

Sanity Check

As this can cause an email loop if not configured correctly, there is a sanity check that you can make to ensure that you have it correct.
On the properties of the contact, click on the tab “Exchange General”. In the email address box, it should say SMTP then username@location.domain.com. If it says username@domain.com then it is wrong and needs to be changed.
On the email addresses tab, the default email address should be @location.domain.com

End Result – Features and Benefits

The net result of this procedure is

Email for your primary domain can be delivered to any server and it will be routed correctly this is useful for backup queuing of email if the other server is down.

Users can type in the full public email address (username@domain.com) from any site and it will be routed correctly.

Users from all sites will appear in the GAL.

You can create distribution lists on all three sites with the same membership.

By using mail enabled contacts the email destined for the other sites is not stored on your server – taking up no storage space on the server.

The sites only need an internet connection – no direct site connection required.

It can take a while to initially configure, but once done, very easy to maintain if you have limited servers.

About Zeros & Ones

Hi, I’m Mohamed Fawzi and I am working as Senior Infrastructure System Engineer for LINK development company. This blog covers Virtualization technology and Cloud Computing.
*All the usual disclaimers are applied :)
The information in this weblog is provided "AS IS" with no warranties, and confers no rights. This weblog does not represent the thoughts, intentions, plans or strategies of my employer. It is solely my opinion.